This application seeks continued support for an established research program in bioinorganic chemistry directed at a detailed molecular understanding of biologically relevant iron-sulfur clusters, various types of which are found at all levels of life. In addition to their classical role as electron carriers, these clusters are now known to function as catalytic sites for redox and non-redox transformations of substrates, and to act as sensors and regulators for certain cell processes. While a general understanding of the two most pervasive cluster types (Fe2S2, Fe4S4) is well developed, there remains issues of cluster formation, interconversion, and reactivity that require further investigation. Research on more complex clusters, some of which contain heterometals such as molybdenum, vanadium, and nickel, is also emphasized. In the enzymes nitrogenase, sulfite reductase, and carbon monoxide dehydrogenase, the catalytic sites are bridged assemblies in which two discrete fragments are spatially disposed by protein structure and linked by one or more covalent bonds. This research seeks elucidation of the pathways of formation, geometrical structures, electronic features, and reactivity properties of clusters and bridged assemblies using the synthetic analogue approach, whose objectives are structural and functional representations of protein-bound cluster sites. In a recent initiative, peptides are utilized as ligands in the synthesis and stabilization of complex metal sites. Among the problems proposed for investigation are the stability of the [Fe4S4]0 oxidation state in the iron protein of nitrogenase, binding properties of cuboidal [Fe3S4]0 clusters toward metal ions, cluster interconversion and the Fe4S4 cluster as a reactant in the catalytic mechanism of biotin synthase, the synthesis and properties of analogues of the Fe8S7 and MFe7S9 (M = Mo, V, Fe) clusters of nitrogenase, and peptide-based analogues of carbon monoxide dehydrogenase and sulfite oxidase. The proposition that designed peptides can be used as scaffolds or templates in the synthesis and stabilization of protein-bound metal sites of their close analogues will be tested. A significant portion of the proposed research is intended to contribute to the developing area of metallocenter synthesis and biosynthesis by showing what synthetic routes are feasible for cluster construction in the absence and presence of peptides and larger proteins.